1. Field of the Invention
The present invention relates to an image processing apparatus and method and, more particularly, to a coding process of moving picture data and still picture data.
2. Description of the Related Art
Electronic still cameras have been studied and commercialized which digitally record a picture on a recording medium such as a semiconductor memory, a magnetic disk, and a magneto-optic disk rather than on a silver film.
If a recording medium is randomly accessible, the pictures recorded are reproduced in an order different from the order of recording, and any recorded picture will be quickly reproduced. A digitally recorded picture is free from degradation in its repeated dubbing operation, and presents scalability along the time axis. The digitally recorded picture, however, requires a large quantity of data compared with its analog counterpart.
As means for reducing the quantity of recorded data, information compression techniques will be useful. A diversity of methods have been proposed including JPEG (Joint Photographic Coding Experts Group) for still pictures and MPEG (Moving Picture Coding Experts Group) for moving pictures.
In the JPEG standard, a value in a spatial coordinate system is subjected to orthogonal transformation (discrete cosine transformation) to convert it into a value on a frequency coordinate, according to a unit of 8.times.8 pixels, and a transformation coefficient is quantized, and variable-length coded using Huffman coding. In this way, the quantity of data is compressed to approximately one-tenth, or less, of the original quantity. Since this process is a semi-decodable coding one, an input signal cannot be perfectly faithfully reproduced, but no problems are practically presented in image quality. To decompress the compressed information, the process in the compression steps is reversed.
Since a priority is placed on the quality of image in still picture taking, some degree of time-consuming postprocessing (including compression process and recording process to a recording medium) will be considered tolerable. In fast serial picture taking, however, the time interval between pictures is dependent on the time required for postprocessing; thus, the time required for compression process and recording process must be shortened or the postprocessing must be partly performed in a parallel fashion to reduce standby time for image pickup.
When a short-time moving picture is recorded, a frame rate of 30 frames/s (or 60 feet/s) or so is required to achieve a smooth motion of pictures.
To this end, the shortening of the time required for postprocessing is also necessary. Furthermore, it is also necessary to both increase the memory capacity of the recording medium and to heighten the image compression rate. To perform compression and recording on a real-time basis, the construction of the apparatus becomes complicated, and the recording medium has to feature a fast write capability.
The MPEG standard, the known compression technique for moving picture, increases the compression rate by adding time axis compression to the compression of each picture. More particularly, there is a high correlation between the frames subsequent to or prior to a reference picture (similar pixel information is derived therefrom) even if some motion is involved therebetween. Differentials from the reference picture values at the respective pixel addresses are determined and coded to further increase the effect of compression. Furthermore, an object to be compared is block matched between comparison frames on a block by block basis. The differential between frames is determined by shifting the object horizontally across pixels and vertically across lines by a distance depending on the motion of the object. The effect of compression is thus further increased.
Although the moving picture compression is thus accomplished to achieve a higher compression rate, the apparatus becomes more complex in construction and more costly accordingly.
As described above, the use of inter-frame correlation in addition to intra-frame compression is particularly important in the moving picture compression process to achieve a high compression rate among other things in connection with the recording of the moving picture.
The random access capability to digitally record a moving picture onto a recording medium in a random access fashion is useful because it also works during the reproduction of the moving picture. If the inter-frame compression is effected, however, the reference frame of a desired frame in inter-frame compression must be first decompressed to reproduce the desired frame, and this involves a complicated reproduction process. Furthermore, in case of a semi-decodable coding process, the image quality will be degraded.
A user may desire to record a high-definition still picture at a particular point in time during continuous recording. In the conventional art, the user cannot perform selective compression process to compress that particular picture at a particularly high-quality level.